The theoretical basis of tumor immunotherapy is that the immune system can identify tumor-associated antigens and regulate the body to attack tumor cells (highly specific cytolysis). In the 1950s, Burnet and Thomas made the theory of “immunological surveillance” that holds that mutational tumor cells that often occur in the body can be identified and eliminated by the immune system, laying a theoretical foundation for tumor immunotherapy [Burnet F M. Immunological aspects of malignant disease. Lancet, 1967; 1: 1171-4]. Then, a host of tumor immunotherapies, including cytokine therapy, monoclonal antibody therapy, adoptive immunotherapy and vaccine therapy, have been applied to clinical practice.
In 2013, CAR-T, a more advanced tumor immunotherapy, was successfully put to clinical use, and showed unprecedented clinical effects. CAR-T is short for Chimeric Antigen Receptor T-Cell Immunotherapy. Clinically, the most leading CAR-T is Novartis' CLT019. For patients with refractory-relapsed acute lymphoblastic leukemia and treated with CLT019, the six-month tumor progression-free survival rate can reach 67%, and the longest response time can be more than two years. By cooperating with hospitals, Shanghai Unicar Biomedical Technology Co., Ltd., a Shanghai-based company, treated 36 patients with refractory-relapsed acute lymphoblastic leukemia, among whom 24 as a percentage of 66.6% experienced complete remission. It's a subversive breakthrough in anti-cancer research. CAR-T may be one of the therapies that are the most likely to cure cancer, and was named the best in top 10 breakthroughs of science and technology 2013 by the journal Science.
In spite of the significant curative effect of CAR-T, in the treatment with the therapy, there may be a special clinical syndrome, of which common clinical manifestations are fever, hypotension, shivering, and a neurological symptom related to a range of significantly elevated cytokine levels in serum and called Cytokine Release Syndrome (CRS). The occurrence mechanism of the syndrome is that after the binding of antigen to T cell receptor, T cell is activated and releases a series of cytokines including IL-6, giving rise to a systematic inflammatory reaction, of which delayed treatment is likely to cause pulmonary edema and then the death of patients (see FIG. 1 for the signaling pathway of IL-6).
Currently, clinically, inflammatory reaction can be inhibited by intravenously injecting antihistamine (such as chlorphenamine maleate), or corticosteroid (such as hydrocortisone); however, correspondingly, CAR-T cell's killing effect on tumor is inhibited, leading to a higher relapse rate of such patients and affecting the curative effect of CAR-T.
Another feasible treatment option is to use commercialized Tocilizumab (Yamero®) to control the occurrence level of CRS. Tocilizumab is human IL-6 receptor monoclonal antibody. The specific binding of Tocilizumab to IL-6 receptor can block IL-6 signal transduction to reduce acute phase reactants, hepcidin products, B cell activation, bone resorption and transformation of cartilage and inhibit the differentiation from T-lymphocyte to Th17 cell to effectively control inflammatory reaction. However, Tocilizumab also has some obvious drawbacks. First, it's very expensive. A Tocilizumab injection for 10 kg of body weight is priced at about RMB 2,000, and an adult patient generally needs 5 ones at a time, which is hard for average families to afford. Second, patients injected with Tocilizumab are vulnerable to infection later in treatment, as their IL-6 receptors are blocked.
Since 1990s, researchers have discovered that double-stranded RNA (“dsRNA”) can be used to inhibit the expression of protein. As such ability of silent gene has great potential in the treatment of human diseases, a great many researchers and commercial entities have invested considerable resources in the development of therapies based on the technology.
From a mechanism perspective, after entering plant and invertebrate cells, dsRNA is broken down by Type III endonuclease Dicer into siRNA. [Sharp, RNA interference-2001, Genes Dev. 2001, 15:485]. Type III endonuclease Dicer breaks dsRNA down into siRNA with 2 base bulges and 3′ sticky ends. [Bernstein, Caudy, Hammond, & Hannon, Role for a bidentate ribonuclease in the initiation step of RNA interference, Nature 2001, 409:363]. In the integration of siRNA with RNA-induced Silencing Complex (RISC), one or more helicase(s) in RISC unwind(s) double-stranded siRNA, making complementary antisense strands direct target recognition. [Nykanen, Haley, & Zamore, ATP requirements and small interfering RNA structure in the RNA interference pathway, Cell 2001, 107:309]. After the integration with corresponding target mRNA, one or more endonuclease(s) in RISC cleave(s) target mRNA, giving rise to mRNA silencing. [Elbashir, Elbashir, Lendeckel, & Tuschl, RNA interference is mediated by 21- and 22-nucleotide RNAs, Genes Dev 2001, 15:188].
Such interference effect can last long and remain effective after cell division. And RNAi has very good sequence specificity. [Kisielow, M. et al. (2002) Isoform-specific knockdown and expression of adaptor protein ShcA using small interfering RNA, J. of Biochemistry 363: 1-5]. Therefore, RNAi can knock down a type of transcript with its specificity and without affecting other mRNAs with similar sequences. Such features enable siRNA system to show its potential and value in the inhibition of gene expression, gene function research and medicine target validation. In addition, siRNA system can be used to treat relevant diseases, including (1) diseases caused by gene overexpression or misexpression and (2) diseases caused by gene mutation.